The invention relates in general to a liquid flow controller which controls a flow of liquid by measuring a pressure drop. More particularly, the invention relates to a liquid flow controller which is relatively insensitive to temperature dependent changes in the viscosity of the liquid that may perturb the sensed pressure drop.
Semiconductor integrated circuits are fabricated by using epitaxy, chemical vapor deposition, plasma etching, and the like in processing chambers. In the course of a number of these wafer fabrication processes it is often necessary to employ reactant materials that normally are liquid at room temperature and atmospheric pressure. For instance, some epitaxial deposition processes employ silicon tetrachloride (SiCl.sub.4) as a silicon source. Silicon tetrachloride is liquid at standard temperature and pressure. It may be converted to vapor in a bubbler or similar apparatus.
In a bubbler, a flow of a relatively non-reactive gas, such as dry nitrogen or argon, is directed beneath the surface of a quantity of reactant liquid, such as silicon tetrachloride. A portion of the liquid silicon tetrachloride is vaporized and entrained in the gas stream as silicon tetrachloride vapor and the accompanying gas-vapor mixture is then metered through a conventional mass flow controller. One of the problems with such conventional metering is that it is often difficult to determine precisely the amount of vapor which is entrained in the gas-vapor stream and thus the amount of vapor flowing into the process chamber. Hence, it is difficult to meter accurately the amount of liquid reactant flowing into the process chamber.
Another reactant that suffers from similar drawbacks is tetraethoxysilane (TEOS) , which is sometimes used in low temperature deposited oxide processes. TEOS, similarly, may be introduced into the process chamber in which it is employed via a bubbler.
Attempts have also been made to use thermal liquid flow controller systems that are akin to the thermal mass flow controller systems employed for the metering of gases and vapors in the semiconductor industry. Such thermal liquid flow controller systems typically allow a liquid to flow through a conduit. The conduit has an upstream temperature sensing element and a downstream temperature sensing element in contact with it. A portion of the conduit is heated and the difference in temperature between the upstream and downstream sensing elements is reflected as a signal that is a flow rate indicator. However, a number of problems have been encountered with such thermal liquid flow controllers. The sensing tubes in such controllers are unstable. They become clogged with crystalline deposits and are rendered inoperable. The sensing tubes also, because of their associated heaters, tend to vaporize the liquid, trapping bubbles in the tube and causing inaccurate flow reading.
Other types of liquid flow metering systems employ differential pressure producers to produce a pressure drop caused by the stream of moving liquid. The pressure drop is then measured and is used to develop a signal indicative of the liquid flow. One of the drawbacks associated with such conventional differential pressure flow meters is that, in particular for use in liquids, the viscosity of the liquid may change as the temperature of the liquid changes. Since the flow rate is a function of the pressure drop as well as the viscosity of the fluid, determinations of the flow rate made solely from the pressure drop alone may prove to be inaccurate if the liquid has a viscosity other than the viscosity at which the instrument has been calibrated. In most applications, however, such changes in viscosity do not result in significant difficulties. The demands of semiconductor processing are such, however, that it is necessary to meter precise quantities of the reactants into the process chamber because failure to do so may result in the production of unacceptable wafers. For instance, changes in the amount of various reactants may result in changes in the amount of doping in the wafer or in in the thickness of deposited oxides, deposited nitrides and the like. As the design rules for integrated circuits have smaller and smaller dimensions, such relatively small changes in reactant flows may result in the production of unacceptable integrated circuits.
What is needed is a liquid flow controller which can precisely meter a known mass of reactant on a mass per unit time basis and can deliver the reactant in a vapor state to the process chamber.